I. Field of the Invention
The present invention pertains to methods and an apparatus useful for determining the type, extent of, and thermal threshold of fouling in heat exchanger elements. The apparatus of this invention is capable of being easily installed and removed from the simulated side arm of various types of heat exchangers thereby facilitating the evaluation of the heat exchanger fouling.
In thermodynamic apparatuses, such as heat exchangers, condensers, and the like, the formation of corrosion products, mineral, and organic deposits from various heating or cooling media can, over time, impair the thermal transmission or thermal resistance of the heat exchanger elements. The formation of corrosion products, mineral deposits, and organic deposits can be counteracted by intermittently cleaning the heat exchanger or through the controlled addition of fouling inhibiting additives to the heat exchange media. Cleaning the heat exchangers or chemical addition can be very expensive. To reduce these expenses, heat exchanger tubes are monitored to establish the presence of and the magnitude of fouling materials, and to evaluate the effectiveness of chemical addition. As a result of the monitoring, manual cleaning or chemical addition can be efficiently controlled.
The methods presently used to evaluate heat exchanger tube fouling rely greatly on balancing the flow rate of the heat exchange media through multiple heat exchanger test tubes. The methods produce inaccurate results when the flow rate of the heat exchanger media is unbalanced. Additionally, the monitorial methods presently used are generally incapable of identifying the type foulant.
II. Description of the Art
U.S. Pat. No. 2,330,599, to Kuehni, describes the basic principles for evaluating the thickness of a material using a thermal testing apparatus. The patent describes a thermal conductivity testing apparatus that includes a heat source and two resistance elements. One resistance element is placed into contact with a plate and the other is left uncontacted. The difference in the temperatures of the two resistance elements is monitored. The rate at which the temperature of the resistance monitor in contact with the plate decreases correlates to the thickness of the plate it touches.
Other patents also describe similar methods of measuring the thermal conductivity of fluids and other materials. These patents include United Kingdom Patents 1,423,830, 1,403,950, and 855,658. The '658 patent describes an apparatus for measuring the thermal conductivity of a test material using two probes mounted within an insulated block where one probe is contacted with the test material while the other probe remains isolated from the test material. The '830 patent describes an apparatus and method for measuring heat flux using a single probe. A single probe is exposed to a heat flux and the rate at which the probe increases in temperature is measured. The '950 patent describes a method for measuring the thermal diffusivity of a sample by exposing a first surface of a sample to heat or radiation source while maintaining a second surface of a sample at a constant temperature. When the first surface is exposed to a heat or radiation pulse, the power necessary to maintain the second surface of a sample at the desired temperature is reduced. The power consumption is then correlated with the heat or radiation pulse magnitude to determine the thermal diffusivity of the sample.
U.S. Pat. No. 2,951,360 describes a method and apparatus for testing the thermal conductivity of materials. The method and apparatus are useful for testing the quality of metal welds.
U.S. Pat. No. 3,724,267 describes a heat flux sensing device. The device includes two thermocouples located at two different locations along the length of a conductor. The thermocouples sense a temperature gradient from which a heat flux can be determined.
U.S. Pat. No. 4,024,751 describes an apparatus for determining the heat transfer efficiency of a heat exchanger wall. The '751 patent recognizes that the efficiency of a heat exchanger is diminished by build up of materials and scale on heat exchanger wall surfaces. The claimed apparatus evaluates the magnitude of scale build up by heating the wall of a heat exchanger tube from first pre-determined temperature to a second pre-determined temperature, halting the heating, and measuring the time it takes for the temperature of the measured portion of the heat exchanger wall to drop from the second predetermined temperature to the first predetermined temperature. The amount of time it takes to return from the second temperature to about the first temperature can be correlated to heat exchanger scale accumulation. The apparatus disclosed is a permanent apparatus including a heating means in direct contact with a heat exchanger tube.
U.S. Pat. No. 4,722,610 describes a monitor for determining the build up of slag on the flame side of water cooled walls of a coal-fired steam generator. The monitor includes a heater located adjacent to a thermocouple in a body. The thermocouple usually monitors the temperature of the body and when the body temperature decreases, this is an indication of slag build up. This indication is confirmed by heating the body with the heater, and measuring the temperature drop of the body using the same thermocouple. A slow drop in the temperature of the body indicates a large build up of slag.
Other apparatuses and methods for evaluating heat exchanger performance are known. However, the art lacks a method that uses a guaranteed clean reference as the basis for evaluating heat exchanger fouling.